Method for polishing a substrate surface

ABSTRACT

According to one aspect of the invention, an improved process for preparing a surface of substrate is provided wherein the surface of the substrate is prepared for a chemical mechanical polishing (CMP) process, the CMP process is performed on the surface of the substrate, and the surface of the substrate is finished to clear the substrate surface of any active ingredients from the CMP process. Also, an improved substrate produced by the method is provided. According to one aspect of the invention, particular polishing materials and procedures may be used that allow for increased quality of AlN substrate surfaces.

RELATED APPLICATION

[0001] This application claims the benefit under Title 35, U.S.C. §119(e) of co-pending U.S. Provisional Application Serial No. 60/331,868,filed Nov. 20, 2001, entitled “Chemical Mechanical Polishing (CMP)Process” by Leo J. Schowalter, J. Carlos Rojo, Javier Martinez Lopez andKenneth Morgan, the contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to the preparation ofsemiconductor substrate surfaces, and more specifically to preparingsubstrate surfaces using chemical mechanical polishing (CMP).

BACKGROUND OF THE INVENTION

[0003] Chemical mechanical polishing (CMP) is a process that is used topolish surfaces of semiconductor wafers. In particular, CMP employs bothphysical and chemical forces to polish wafer surfaces. First, a loadforce is applied to the back of a wafer while it rests on a polishingpad. Both the pad and the wafer are then counter-rotated while slurrycontaining both abrasives and reactive chemicals is passed underneath.

[0004] CMP is typically used to planarize a surface of a wafer for thepurpose of creating an integrated circuit. CMP has emerged as theplanarization method of choice because of its ability to planarize overlonger lengths than traditional planarization techniques. As discussed,CMP is a combination of a chemical reaction and mechanical action.First, the slurry weakens the wafer surface and the slurry particles, inaddition to particles located on the pad, complete material removal fromthe wafer surface.

[0005] It is desired to use aluminum nitride (AlN) as a substratematerial for creating commercial semiconductor devices, however, thereare problems in processing AlN surfaces to derive substrate surfacessuitable for epitaxial growth. For example, current CMP processes damagethe AlN material making it unsuitable for epitaxial growth. It would bebeneficial to have an improved method for polishing AlN wafers thatproduces a quality surface suitable for epitaxial growth.

SUMMARY OF THE INVENTION

[0006] One illustrative embodiment of the invention is directed to amethod of preparing a surface of a substrate, the method comprising theacts of preparing the surface of the substrate for a chemical mechanicalpolishing (CMP) process, performing the CMP process on the surface ofthe substrate, and finishing the surface of the substrate, wherein theact of performing the CMP process further comprises an act of applyingan abrasive suspension in a solution during the CMP process. Accordingto one aspect of the invention, a method is provided wherein thesubstrate is aluminum nitride (AlN).

[0007] According to another aspect of the invention, the abrasivesuspension in the solution further comprises a silica suspension in ahydroxide (basic) solution. According to another aspect of theinvention, the abrasive suspension includes a KOH-based slurry.According to one aspect of the invention, the abrasive suspension in thesolution further comprises a silica particles suspended in anammonia-based slurry.

[0008] According to another aspect of the invention, the act ofperforming the CMP process includes an act of applying the abrasivesuspension at a rate of approximately 0.5 mL per minute for a circular8.0″ diameter polishing surface. According to one aspect of theinvention, the act of performing the CMP process includes an act ofmaintaining a polishing speed of the sample in a range of approximately13 to 18 m/sec.

[0009] According to another aspect of the invention, the act ofpreparing includes cleaning a polishing apparatus prior to polishing thesurface of the substrate to substantially reduce contamination of thepolishing surface. According to one aspect of the invention, the act offinishing the surface of the substrate includes an act of substantiallyrinsing the abrasive suspension from the surface of the substrate.According to another aspect of the invention, the method furthercomrpises cleaning, during the preparing acts, the surface of thesubstrate with a solvent. According to another aspect of the invention,the substrate is an on-axis, Al-polarity, c-face surface, and theabrasive suspension has a pH value of approximately 10.5 or greater.According to another aspect of the invention, an substrate is providedproduced by the method. According to another aspect of the invention, adevice having a substrate produced by the method is provided.

[0010] According to another aspect of the invention, a method isprovided for preparing a surface of a substrate. The method comprisesacts of preparing the surface of the substrate for a chemical mechanicalpolishing (CMP) process, the act of preparing comprising acts ofdetermining an orientation of the substrate; and performing a removal ofthe surface of the substrate based on the orientation. According toanother aspect of the invention, the substrate is AlN, and the act ofdetermining an orientation includes an act of determining, for the AlNsubstrate, at least one face of the AlN substrate and its orientation tothe surface to be polished. According to another aspect, the act ofperforming a removal includes an act of determining a period of removalbased on the orientation.

[0011] According to another aspect of the invention, the act ofperforming a removal includes an act of removing exposed off-axismaterial from the substrate surface. According to another aspect, theact of removing the exposed off-axis material includes removing between50 and 100 μm of material from the substrate surface.

[0012] According to another aspect, the substrate includes a surfacenormal to be polished, and the act of determining an orientationincludes determining, for a given face of the substrate, an orientationof the normal surface with respect to the given face. According toanother aspect of the invention, the act of performing a removal of thesurface of the substrate based on the orientation of the normal surfacewith respect to the given face.

[0013] According to another aspect, the substrate includes anAl-polarity side of c-face substrate, and the act of performing aremoval includes performing at least one of a dry grinding and polishingusing a polymer diamond suspension to prepare the Al-polarity side ofthe c-face substrate. According to another aspect of the invention, theAlN substrate includes a non-polar surface, and the act or performing aremoval further comprises an act of removing between 10 and 20 μm of thenon-polar surface.

[0014] According to another aspect of the invention, the method furthercomprises an act of performing the chemical mechanical polishing (CMP)process, and a polishing of the substrate surface is performed based onthe orientation. According to another aspect, the polishing furthercomprises an act of polishing the substrate surface with a slurry havinga pH value, and the slurry being selected based on the face of thesubstrate and the pH value.

[0015] According to another aspect, the substrate is AlN, and thesubstrate surface is the on-axis Al-polarity, c-face surface, and the pHvalue of the selected slurry is greater than 10.5. According to anotheraspect of the invention, an substrate is provided produced by themethod. According to another aspect of the invention, a device having asubstrate produced by the method is provided.

[0016] Further features and advantages of the present invention as wellas the structure and operation of various embodiments of the presentinvention are described in detail below with reference to theaccompanying drawings. In the drawings, like reference numerals indicatelike or functionally similar elements. Additionally, the left-most oneor two digits of a reference numeral identifies the drawing in which thereference numeral first appears.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention is pointed out with particularity in the appendedclaims. The above and further advantages of this invention may be betterunderstood by referring to the following description when taken inconjunction with the accompanying drawings in which similar referencenumbers indicate the same or similar elements.

[0018] In the drawings,

[0019]FIG. 1 shows a conventional CMP apparatus that may be used topolish substrate surfaces in a CMP process according to one embodimentof the invention;

[0020]FIG. 2 shows a flow chart of a process used to polish a substratesurface according to one embodiment of the invention;

[0021]FIG. 3 shows a more detailed process for polishing a substratesurface according to one embodiment of the invention;

[0022]FIG. 4 shows an AFM image of an Al-polarity c-face of an AlNsubstrate after performing CMP polishing according to one embodiment ofthe invention; and

[0023]FIG. 5 shows an AFM image of an AlN substrate surface tilted 45degrees with respect to a c-axis direction of the AlN substrate, thesurface pictured after performing CMP polishing according to oneembodiment of the invention.

DETAILED DESCRIPTION

[0024] The use of single crystal III-nitrites allow improved epitaxialgrowth, improved thermal and chemical compatibility, as well as improvedthermal conductivity. Applications of wide-bandgap and high-temperaturesemiconductors including the development of blue/UV solid-state chargeinjection lasers, UV optical sources and detectors, high power microwavedevices, high power switches, and high temperature applications.However, there is a need to obtain a method for suitably preparingsurfaces of aluminum nitrites (AlN) substrates for epitaxial growth toproduce these devices. For example, nitrites (e.g. AlN, Al_(x),Ga_((1−x))N, etc.) may be epitaxially grown on single crystal AlNsubstrates by, for example, Organometallic Vapor Phase Epitaxy (OMVPE)and other formation processes.

[0025] Device fabrication on AlN substrates generally involves theepitaxial growth of a device layer. The quality of this device layerdepends on the quality of the surface of the AlN substrate. Despite thebest polishing techniques, the wafer surface can retain polishingdefects such as micro-damage and surface roughness. Surface defects areknown to effect the quality of finished semiconductors prepared throughepitaxial growth. It is believed that conventional polishing proceduresdamage the AlN surface both at a submicron scale and just under the AlNsurface in such a way that the substrate is unsuitable for epitaxialgrowth. Difficulty arises in preparing such surfaces because the AlNsurface is unable to be polished using conventional CMP process withoutintroducing surface defects. Because conventional CMP techniques damagethe AlN surface, a method that produces an atomically smooth surfacesuitable for epitaxial growth for AlN crystals is desired.

[0026] According to one aspect of the invention, an improved process forpreparing a surface of substrate is provided wherein the surface of thesubstrate is prepared for a chemical mechanical polishing (CMP) process,the CMP process is performed on the surface of the substrate, and thesurface of the substrate is finished to clear the substrate surface ofany active ingredients from the CMP process. Also, an improved substrateproduced by the method is provided. According to one aspect of theinvention, particular polishing materials and procedures may be usedthat allow for increased quality of AlN substrate surfaces.

[0027] To employ AlN crystals as substrates for epitaxial growth ofother compound semiconductors such as GaN, InN, or 2H—SiC, large area,high perfection AlN wafers are needed. After growing AlN boules, theboules are generally cut into wafers using annular diamond saws or wiresaws and then the surface of each is ground and polished flat. Accordingto one embodiment of the invention, AlN crystal surfaces (e.g., the a,+c or −c surfaces of AlN) are polished using CMP to derive surfacessuitable for epitaxial growth. Optimally, these polished surfaces areflat, highly perfect surfaces with a minimum of scratches, pits, ordamage dislocations and other subsurface damage. Also, these polishedsurfaces are also free of aluminum oxide coating or islands.

[0028] In any grinding process, the powder employed for grinding shouldhave a microhardness greater than or equal to that of the crystal beingground. The basic goal is to remove material from the crystal in a shortperiod of time. The polishing is intended to give a mirror-like finishto the surface, and leave a damage-free crystal underneath. Thispolishing process is much like “chemical-mechanical-planarization”processes as are known in the art which are used most frequently toplanarize wafer surfaces of multilayer devices. In this process, thepolishing lubricant actually reacts slowly with the crystal beingpolished. The slurry and abrasive pad are designed to continuouslyremove the reaction products without appreciably damaging the underlyingcrystal.

[0029] In CMP, high elevation features of a wafer are selectivelyremoved (i.e., material from high elevation features is removed morerapidly than material at lower elevations), resulting in a surfacehaving an improved planarity. Mechanical polishing, assisted by chemicalaction, produces such selective material removal. The process isperformed by mounting the wafer face down on a carrier. The carrier isthen pressed against the rotating platen containing a polishing pad. Thecarrier itself is also rotated. An abrasive-containing aqueous slurry isdripped onto the table, saturating the pad. Conventionally, the slurrycomprises abrasives of silica, alumina, cerium, or other nanometer-sizeparticles suspended in an alkaline or acidic medium.

[0030] According to one aspect of the invention, it is realized thatorientation affects mechanical preparation of a substrate surface priorto CMP processing. So-called c-face substrates are produced when the AlNcrystal is sliced perpendicular to the c-axis of the crystal. Thesec-face substrates are polar and the surfaces on the two sides of thesubstrate will have quite different properties. One side of thesubstrate is aluminum (Al) terminated (or so-called Al-polarity, c-face)and the other side is nitrogen (N) terminated (or so-called N-polarity,c-face).

[0031] When the AlN crystal is cut so that the c-axis of the crystal iscontained in the plane of the substrate, a non-polar substrate isproduced wherein the two surfaces have identical behavior. Substratesthat are cut at some angle away from the c-axis (other than 90°) alsodemonstrate some polarity effects, but these effects are not as strongas the effects of c-face substrates. C-face substrates are currentlybeing used to produce electronic devices that take advantage of polareffects to create a high-density electron gas without any or withminimal doping. However, optoelectronic devices are expected to bebetter if non-polar substrates are used.

[0032] According to one embodiment of the invention, it is realized thatsubstantial differences exist for optimal preparation of the substratessurfaces with different crystallographic orientations. In the case of anAlN substrate, it is realized that the Al-terminated, c-face is notreactive with water, but the N-terminated c-face is reactive with water,along with non-polar faces. During wet lapping and polishing, theAl-polarity face tends to chip under the same conditions that arewell-suited to mechanically polish the non Al-polarity faces and forAl-polarity faces where the c-axis is oriented 20 degrees or more awayfrom the surface normal of the substrate. (Effectively, wet mechanicalprocessing used on orientations other than the on-axis c-face isactually a chemical mechanical processing with a fixed abrasive).According to one embodiment of the invention, dry grinding and polymerdiamond suspensions are used to prepare the Al-polarity side of c-facesubstrates that are close to on-axis (misorientation less thanapproximately 20°). Even under strictly mechanical polishing, theremoval rates observed for the on-axis, Al-polarity surface is lowerthan for all other crystal orientations.

[0033] According to one embodiment of the invention, it is realized thatthe amount of material that should be removed during the mechanicalprocessing prior to fine mechanical polishing and CMP depends uponquality of the saw cut. Typically between 50 and 100 μm must be removedto level the irregular cut of the saw for an annular saw. At this point,the sample is generally planar and should have a low pit density, andthe sample may be subjected to fine mechanical processing. In this step,it is realized, according to one embodiment, that planarity of thesamples be brought to the highest level for c-face, Al-polaritysubstrate surfaces, because it is realized that any exposed off-axismaterial will lead to undercutting of the desired surface during CMP.For non-polar surfaces, the high chemical reactivity allowsplanarization during CMP (few microscopically viewable pits andscratches are allowable and does not cause the CMP undercutting effectson the Al-polarity surfaces. Fine mechanical processing generallyinvolves a measured removal totally between 10 and 20 μm of samplethickness.

[0034] According to one embodiment of the invention, it is realized thatthe removal rate during CMP is very much a function of thecrystallographic orientation of the substrate surface. For theAl-polarity, c-face substrates, the removal rate increases from around 1μm per hour to over 10 μm per hour as the angle between the surfacenormal and the c-axis is increased from near zero to over 20°. For thenon-polar and N-polarity, c-face surfaces, the removal rate ismechanically limited by the abrasive particles in the slurry.

EXAMPLE

[0035]FIG. 1 shows a conventional CMP apparatus that may be used topolish substrate surfaces in a CMP process according to one embodimentof the invention. A wafer 101 to be polished is mounted on a wafercarrier 102 which generally includes a backing film 109 positionedbetween wafer 101 and a chuck 108 which holds the wafer and the wafercarrier 102. Wafer carrier 102 is rotated by a spindle 105. A force isapplied to the wafer carrier 102 to contact the wafer 101 with one ormore polishing pads 103A-103B. The one or more polishing pads 103A-103Bare adhered to a platen 106 which also rotates. Further, a slurry 107 isapplied to the pad 103A-103B and the drip rate is controlled, forexample, by a control flow dispenser (not shown).

[0036] It should be appreciated that other CMP tools and/or polishingapparatus may be used and the invention is not limited to any particularCMP tools or polishing configurations.

[0037] According to one aspect of the invention, the CMP process mayinvolve polishing the substrate using a slurry, the slurry comprising anabrasive suspension in a solution such that the slurry is capable ofetching the substrate surface and creating a finished surface suitablefor epitaxial growth. For example, a silica suspension in a solution maybe used. This solution may be, for example, a hydroxide (basic)solution. Such a slurry is available commercially, for example, as theKOH-based CMP slurry known in the art as SS-25 (Semi-Sperse 25),available from Cabot Microelectronics or the Syton slurry available fromMonsanto.

[0038] According to one embodiment of the invention, it is realized thata higher pH KOH slurry works better for the Al-polarity, c-face surface.The SS25 slurry has a pH of 11.0. Its high pH distinguishes it fromother commercially availably KOH slurries (like Syton and Glanzox) whichhave lower pH. For the on-axis Al-polarity, c-face surface, it isrealized, according to one embodiment of the invention, that the pHshould be over 10.5 to obtain observable removal rates.

[0039] It should be appreciated that other slurry types may be used. Forexample, diamond, silicon carbide, or other material may be used in aslurry. Also, other commercially-available slurries are available (e.g.,SS-25-E and SS-225 (ammonia hydroxid-based) both of which are availablefrom Cabot Microelectronics, AM 100 and Rodel 2371 (ammonia-basedslurries)).

[0040] Such chemical/mechanical polishing methods are particularlysuitable for preparing very hard surfaces, such as aluminum nitride(AlN) surfaces. It should be appreciated that various aspects of theinvention reduce an amount of impurities and surface defects introducedinto the AlN crystal substrate by the polishing procedure.

[0041]FIG. 2 shows a process 200 for polishing a substrate according toone embodiment of the invention. At block 201, process 200 begins. Atblock 202, the substrate is prepared for polishing. This may includeremoval of contamination from the surface of the substrate, thepolishing apparatus and environment. Such contamination is generallyresponsible for the introduction of surface defects during the polishingprocess. At block 203, a CMP process is performed on the substratesurface. In the case of AlN, the CMP process may include polishing thesurface using a slurry having a hydroxide solution. For example, theSS-25 slurry available from Cabot Microelectronics may be used. Itshould be appreciated that the invention is not limited to the type ofslurry, and that other appropriate slurries may be used as discussedabove. At block 204, the polished sample is finished. This may involve,for example, removal of the sample from the polishing apparatus andcarefully cleaning the sample to remove any reactive agents introducedby the polishing process. At block 205, process 200 ends.

[0042]FIG. 3 shows a more detailed process 300 for polishing a substrateaccording to one embodiment of the invention. At block 301, process 300begins.

[0043] According to one embodiment of the invention, the AlN substrateis prepared for a CMP process. More particularly, the environment andwafer are cleaned of possible contaminants, and the polishing apparatusadjusted to apply the correct polishing force to the wafer. Thefollowing example is a procedure for preparing the substrate accordingto one embodiment of the invention:

[0044] I. Preparation of Substrate and Apparatus for CMP Process

[0045] 1. The sample surface and mounting block should be cleaned ofsubstantially all possible contamination particles (e.g., wax, dirty,and larger mechanical grit particles). This is typically done with, forexample, organic solvent (e.g., by acetone and/or methanol) cleaningperformed at block 302. Also, any remaining particles from previousprocessings (e.g., CMP fluid residue) should be cleaned from allsurfaces that contact the polishing area so as to reduce the likelihoodof contaminating the polishing surface and potentially damaging thesubstrate surface.

[0046] 2. The mounting block is then prepared to hold the sample atblock 303. For example, the sample may be adhered to a surface of themounting block. The mounting block is then switched from the“mechanical” process fixture to the “CMP” process fixture. According toone embodiment of the invention, these fixtures may be identical items;their use may be segregated to avoid cross contamination between themechanical and chemical mechanical polishing.

[0047] 3. The mounted sample may have the vertical offset corrected toallow the sample appropriate contact with the polishing surface. A massto ensure proper down force can be added to the block at 304. Pressurein a range of 5×10⁵ to 7×10⁵ N/m² have been shown to be suitable. Theseare conditions that have yielded the most consistent results for bothnon-polar and polar faces. However, it should be appreciated that theinvention is not limited to any particular set of conditions.

[0048] According to one embodiment of the invention, a CMP process maybe performed on the prepared substrate surface. In one aspect of theinvention, SS-25 (Semi-Sperse 25) slurry available from the CabotMicroelectronics Corporation may be used to polish an AlN substrate.

[0049] II. CMP Process

[0050] The following is a detailed sequence of steps, according to oneembodiment of the invention, to prepare an AlN substrate surface and thedrip rates for a CMP process according to one embodiment of theinvention.

[0051] A new “Multitex 1000” fine polishing pad or any other suitablefine polishing pad should be applied to the polishing table (platen).Generally, such a polishing pad has a self-adhesive backing to adhere itto the platen surface. At block 305, the polishing pad is adhered to theplaten surface. For best results, the polishing pad should be applied tothe polishing deck in such a way to ensure that the pad is not placedwith space behind the pad causing it to be uneven. For example, thepolishing pad may be rolled onto the table from the lead edge to theother side with a cleaned rod so that spaces behind the pad can beavoided.

[0052] At block 306, the pad surface may be cleaned. For example, thepad may be cleaned with distilled water, saturated with water and theempty pad and platen surface rotated (e.g., 45-60 RPM or 13 to 18 m/secin the sample path) on the pad for 10 to 15 minutes. During thiscleaning process, the sample (with the substrate to be polished) shouldbe lifted out of contact with the polishing surface. During this time,the surface of the pad should be thoroughly rinsed with filtereddistilled water. This is performed to ensure that both the pad and thefixture have been completely cleaned of all dust and cross-contaminatedparticles.

[0053] At block 307, the slurry drip rate and rotation of the fixtureshould be set for polishing. During this rotation cleaning of thesurface, the drip rate should be set using an appropriate flow controldispenser. The rate that has yielded the best and most consistentpolishing results has been 3 drops of SS-25 slurry and distilled waterper minute (approximately 0.5 mL/minute of each of the CMP fluid anddistilled water for a circular 8.0″ diameter polishing surface). Itshould be appreciated, however, that the invention is not limited to anyparticular drip rate, and that drip rate depends on the type of slurryused, pressure applied to the sample, and rotation speed. With thisaverage drip rate set, the SS-25 slurry should be allowed to drip ontothe polishing surface allowing the lapping fixture to evenly spread thesolution and fill the sample area with a consistent level of abrasiveparticles and reactive solution prior to polishing the substratesurface. The polishing pad may be preconditioned in this manner prior topolishing for approximately 30-60 minutes or any other appropriate timeto allow for substantially even distribution of slurry and etchant overthe surface of the pad.

[0054] Following the proper rate confirmation and with the observationof proper rotation of the fixture, then the sample may be lowered on thesurface to begin the CMP process. At block, 308, the sample is polished.

[0055] Although a typical process time (polishing time) for this step isapproximately 1 hour, a process time in a range of 5 minutes to 100hours has been shown to adequately polish a surface. It should beunderstood that the invention is not limited to a particular processtime, but rather, the process time range is provided by way of example.

[0056] Although the material removal depends highly upon the crystalquality and the orientation of the sample, common values range from 0.1μm (Al face c-axis) to 60 μm (N face c-axis) for 1 hour processing.

[0057] According to one embodiment of the invention, the sample iscleaned to remove any contamination from the CMP process, and to removetraces of reacting chemicals from the substrate surface to stop anyresidual reactions with the substrate.

[0058] III. Finishing

[0059] 1. At block 309, the sample is rinsed. At the end of the CMPprocess, the sample should be lifted from the polishing surface andimmediately rinsed, for example, with distilled water. This is performedto ensure that the reaction between the AlN and any of the activeingredients in the slurry be stopped, and to remove abrasives that maydry to the prepared surface.

[0060] 2. Following this rinsing, the sample is carefully cleaned atblock 310 with suitable organic solvents to remove the agent bonding thesubstrate to the substrate holder. Mounting waxes soluble in thesesolvents and other common sample holding techniques may be used to holdthe substrates, as well as other adhesive film and vacuum chuckingtechniques. Each of these holding techniques may need differentsubstrate debonding procedures at block 311, as is well-known in theart.

[0061] 3. Following the CMP processing, the sample surface should becleaned very well at block 312. For example, the sample surface may becleaned prior to removal of the sample from the mounting block, and thenthe entire sample may be cleaned when removed.

[0062] 4. At block 311, process 300 ends.

[0063]FIG. 4 shows an AFM image of the Al-polarity of the c-facesubstrate after polishing according to one embodiment of the invention.The substrate surface is oriented with its normal approximately 5° offthe direction of the c-axis. It can be seen that the surface is nearlyatomically flat. Analysis of a wider area shows that all mechanicaldamage has been removed. Similarly smooth surfaces may be obtained forthe N-polarity of a c-face substrate.

[0064]FIG. 5 shows an AFM image of a substrate after polishing accordingto one embodiment of the invention. The surface is tilted 45° withrespect to the c-axis direction. As can be observed, the substratesurface is nearly atomically flat. Evidence of mechanical damage hasbeen removed. The crossed box on the center was eliminated from theimage for to the statistical analysis.

[0065] Having described several embodiments of the invention in detail,various modifications and improvements will readily occur to thoseskilled in the art. Such modifications and improvements are intended tobe within the spirit and scope of the invention. Accordingly, theforegoing description is by way of example only, and is not intended aslimiting. The invention is limited only as defined by the followingclaims and the equivalents thereto.

What is claimed is:
 1. A method of preparing a surface of a substrate,the method comprising the acts of: preparing the surface of thesubstrate for a chemical mechanical polishing (CMP) process; performingthe CMP process on the surface of the substrate; and finishing thesurface of the substrate, wherein the act of performing the CMP processfurther comprises an act of applying an abrasive suspension in asolution during the CMP process.
 2. The method according to claim 1,wherein the substrate is aluminum nitride (AlN).
 3. The method accordingto claim 1, wherein the abrasive suspension in the solution furthercomprises a silica suspension in a hydroxide (basic) solution.
 4. Themethod according to claim 3, wherein the abrasive suspension includes aKOH-based slurry.
 5. The method according to claim 1, wherein theabrasive suspension in the solution further comprises a silica particlessuspended in an ammonia-based slurry.
 6. The method according to claim1, wherein the act of performing the CMP process includes an act ofapplying the abrasive suspension at a rate of approximately 0.5 mL perminute for a circular 8.0″ diameter polishing surface.
 7. The methodaccording to claim 6, wherein the act of performing the CMP processincludes an act of maintaining a polishing speed of the sample in arange of approximately 13 to 18 m/sec.
 8. The method according to claim1, wherein the act of preparing includes cleaning a polishing apparatusprior to polishing the surface of the substrate to substantially reducecontamination of the polishing surface.
 9. The method according to claim1, wherein the act of finishing the surface of the substrate includes anact of substantially rinsing the abrasive suspension from the surface ofthe substrate.
 10. The method according to claim 1, further comprising:cleaning, during the preparing acts, the surface of the substrate with asolvent.
 11. The method according to claim 2, wherein the substrate isan on-axis, Al-polarity, c-face surface, and the abrasive suspension hasa pH value of approximately 10.5 or greater.
 12. A substrate produced bythe method according to claim
 1. 13. A device having a substrateproduced by the method according to claim
 1. 14. A method for preparinga surface of a substrate, the method comprising acts of: preparing thesurface of the substrate for a chemical mechanical polishing (CMP)process, the act of preparing comprising acts of: determining anorientation of the substrate; and performing a removal of the surface ofthe substrate based on the orientation.
 15. The method according toclaim 14, wherein the substrate is AlN, and the act of determining anorientation includes an act of determining, for the AlN substrate, atleast one face of the AlN substrate and its orientation to the surfaceto be polished.
 16. The method according to claim 15, wherein the act ofperforming a removal includes an act of determining a period of removalbased on the orientation.
 17. The method according to claim 14, whereinthe act of performing a removal includes an act of removing exposedoff-axis material from the substrate surface.
 18. The method accordingto claim 17, wherein the act of removing the exposed off-axis materialincludes removing between 50 and 100 μm of material from the substratesurface.
 19. The method according to claim 14, wherein the substrateincludes a surface normal to be polished, and the act of determining anorientation includes determining, for a given face of the substrate, anorientation of the normal surface with respect to the given face. 20.The method according to claim 19, wherein the act of performing aremoval of the surface of the substrate based on the orientation of thenormal surface with respect to the given face.
 21. The method accordingto claim 14, wherein the substrate includes an Al-polarity side ofc-face substrate, and the act of performing a removal includesperforming at least one of a dry grinding and polishing using a polymerdiamond suspension to prepare the Al-polarity side of the c-facesubstrate.
 22. The method according to claim 15, wherein the AlNsubstrate includes a non-polar surface, and the act or performing aremoval further comprises an act of removing between 10 and 20 μm of thenon-polar surface.
 23. The method according to claim 14, wherein themethod further comprises an act of performing the chemical mechanicalpolishing (CMP) process, and a polishing of the substrate surface isperformed based on the orientation.
 24. The method according to claim23, wherein the polishing further comprises an act of polishing thesubstrate surface with a slurry having a pH value, and the slurry beingselected based on the face of the substrate and the pH value.
 25. Themethod according to claim 24, wherein the substrate is AlN, and thesubstrate surface is the on-axis Al-polarity, c-face surface, and the pHvalue of the selected slurry is greater than 10.5.
 26. A substrateproduced by the method according to claim
 14. 27. A device having asubstrate produced by the method according to claim 14.